It is desirable to deposit pyrolitic carbon coatings on certain objects. For example, uranium particles can be coated with pyrolitic carbon which can form a pressure-retentive shell allowing the coated particles to be fabricated into fuel rods for use in nuclear reactors. Another important use for such coatings is for heart valves and other biomedical components because a pyrolitic carbon coating is highly biocompatible and non-reactive with blood.
Pyrolitic carbon is usually deposited on an object by thermally decomposing gaseous hydrocarbons or other carbonaceous substances in vaporous form in the presence of the object. A fluidized bed usually comprises a reaction chamber or furnace surrounded by heat tubes which raise the interior temperature of the furnace to between 1200.degree. and 2000.degree. centigrade. A stream of gas comprising an inert fluidizing gas and a carbonaceous substance is forced through the reaction chamber and through a bed containing both primary objects to be coated and ancillary small particles, usually of a size measured in microns. Such apparatus and improvements thereon are described in U.S. Pat. No. 3,977,896; 4,546,012 and 4,594,270, which are commonly assigned. The teachings and disclosures of these patents are incorporated herein by reference.
Gases are inserted into the reaction chamber through a nozzle. Various kinds and forms of nozzles have heretofore been proposed and used. Examples can be found in U.S. Pat. No. 4,080,927; 4,262,039; 4,271,207 and 4,387,120. In general, these patents address problems associated with achieving a uniform flow through the reaction chamber or other optimum conditions for producing a particular type or condition of carbon coating. Because of the numerous variables involved, different types and characteristics of coating can be obtained by varying conditions. There remains, therefore, a need for apparatus which produce coatings having certain optimized characteristics, selected from the range of possible characteristics. In particular, for carbon coatings for biomedical components, coatings with high-hardness, controlled silicon content and improved porosity (that is, fewer pores in the coating) are desirable for certain parts. I have invented, therefore, a fluidized bed which combines a uniform heat distribution and a multi-jet nozzle to produce a carbon coating having improved characteristics with respect to hardness, porosity and silicon content.